Polarization rotator for head-up display

ABSTRACT

A head up display arrangement for a motor vehicle includes a head up display module having a picture generation unit emitting a light field. A plurality of linear polarizers are arranged in a stack. A first of the linear polarizers receives the light field from the picture generation unit. A last of the linear polarizers emits the light field. A windshield reflects the light field from the last linear polarizer such that the reflected light field is visible to a human driver of the motor vehicle as a virtual image.

CROSS-REFERENCED TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/445,563, filed on Jun. 19, 2019, which currently under allowance,which claims the benefit of U.S. Provisional Application No. 62/687,371,filed on Jun. 20, 2018, which the disclosure of which are herebyincorporated by reference in their entirety for all purposes.

FIELD OF THE INVENTION

The disclosure relates to a head up display (HUD) in a motor vehicle.

BACKGROUND OF THE INVENTION

A head up display emits light that reflects from the front windshield tobe seen by the driver. The light appears to come from a virtual image infront of the driver and in front of the windshield. This type of head updisplay is currently commercially available.

Conventional head up displays create the virtual image by first using adisplay to create an image. Next, the light from the image is reflectedfrom one or more mirrors. Next, the light from the mirrors is reflectedfrom the windshield. The mirrors are designed and positioned relative tothe display so that the light seen by the driver, which is reflectedfrom the windshield, appears to come from a virtual image that isoutside of the vehicle. The mirrors and display are typically containedin a package that occupies a volume beneath the top surface of thedashboard.

With a conventional windshield HUD, the driver needs to view a virtualimage created by reflecting light from the windshield. The image sourceis a projector beneath the top of the instrument panel. The light isincident on the windshield near the Brewster angle, at which allp-polarized light is transmitted, so the reflection of s-polarized lightis much higher than for p-polarized light. However, drivers often wearpolarized sunglasses to avoid seeing reflected light from water on theroad. The polarized sunglasses are oriented to primarily transmitp-polarized light. Thus, the HUD cannot simply project s-polarized lightand still enable the driver to see the reflected light from thewindshield.

Many current owners of HUD-equipped vehicles have expressed unhappinesswith inability to view the HUD image while wearing polarized sunglasses.The following methods are known to enable the image of a windshield HUDto be viewed with polarized sunglasses: (1) The HUD projector emitscircularly polarized or elliptically polarized light. This is not afully satisfactory solution since the windshield reflectivity top-polarized light is small; (2) The driver can wear non-polarizingsunglasses; (3) A plastic lens, including a wave plate, can be clippedover the polarizing sunglasses; and (4) An interior surface of thewindshield can be coated with a reflective layer. Doing so increaseswindshield reflectance, and, as a result, increases veiling glare.

Currently, virtual images produced by head up displays become extremelydim when the user is wearing polarized sun glasses. In some instances,the virtual image completely disappears because the light coming towardsthe driver has a majority of its light in one polarization state.

There is no known solution to the problem of a dim virtual image. Adriver wearing polarized sunglasses cannot see the virtual imageproduced by the HUD on a sunny day.

Circularly polarizing the light after the picture generation unit (PGU)may help slightly, but it does not solve the problem. It is known for awave plate to be used to rotate the linear polarization direction of thelight transmitted through the LCD or convert the transmitted light tocircularly polarized light. Currently available retardation film thatwould be used for this application does not meet the temperaturerequirement for a HUD.

It is also known to design the LCD so the light that exits is linearlypolarized with the desired polarization direction. However, to do sointroduces a long lead time and the resultant LCD is a custom displaythat costs more.

P-polarized light has an electric field parallel to the plane ofincidence. The plane of incidence is a plane which includes thedirection of travel of the light and a vector perpendicular to thesurface from which the light is being reflected. In contrast,S-polarized light has an electric field in the orthogonal direction,perpendicular to the plane of incidence.

SUMMARY

The present invention may provide a head up display (HUD) system thatreflects light from the front windshield of a vehicle to be seen by thedriver as a virtual image. In one embodiment, the system includes atleast one additional optical element which enables the driver to see thevirtual image when he is wearing polarized sunglasses.

The present invention may enable a head-up display (HUD) to provide avirtual image that is easily visible to a driver wearing polarizedsunglasses, while displaying a bright image with the intended colors toa driver not wearing sunglasses. The invention may use a sequence oflinear polarizers to rotate the linear polarization direction of lightcoming from a liquid crystal display (LCD). The invention may make itpossible for a HUD to provide a virtual image with colors that closelymatch the colors provided by an LCD. The invention may make it possibleto quickly tune the fraction of the light exiting the HUD that primarilycontributes to image brightness as seen by a driver without sunglasses(s-polarized), as opposed to the fraction that contributes to imagebrightness as seen by a driver wearing polarized sunglasses(p-polarized).

The invention may convert a fraction of the s-polarized light from a LCDin the HUD to p-polarized light so a driver wearing polarized sunglassesis still able to see the virtual image created by reflecting light fromthe windshield. This invention may provide this function whilemaintaining the intended colors to be viewed by the driver.

In one embodiment, the invention comprises a head up display arrangementfor a motor vehicle, including a head up display module having a picturegeneration unit emitting a light field. A plurality of linear polarizersare arranged in a stack. A first of the linear polarizers receives thelight field from the picture generation unit. A last of the linearpolarizers emits the light field. A windshield reflects the light fieldfrom the last linear polarizer such that the reflected light field isvisible to a human driver of the motor vehicle as a virtual image.

In another embodiment, the invention comprises a head up display method,including providing a head up display module including a picturegeneration unit and a plurality of linear polarizers arranged in astack. Light is emitted by use of the picture generation unit. The lightfrom the picture generation unit is sequentially passed through each ofthe linear polarizers to thereby change a linear polarization directionof the light. The light that has passed through the linear polarizers isreflected off of a windshield such that the reflected light is visibleto a human driver of a motor vehicle as a virtual image.

In yet another embodiment, the invention comprises a head up displayarrangement for a motor vehicle, including a head up display modulehaving a picture generation unit emitting light. A plurality of linearpolarizers receive the light from the picture generation unit,sequentially change a linear polarization direction of the receivedlight, and emit the light with a changed linear polarization direction.A windshield reflects the light from the linear polarizers such that thereflected light field is visible to a human driver of the motor vehicleas a virtual image.

An advantage of the present invention is that it solves the problem ofthe driver not being able to see the HUD virtual image while wearingordinary polarizing sunglasses.

A further advantage of this invention is that reasonably pricedpolarizer material is available that withstands the upper temperaturethat a HUD must be able to withstand (105° C.). Such polarizer materialis used in LCD construction. However, the upper temperature limit forpolymer retarder film that can be used for a wave-plate is typicallyonly 85° C. There are other means of constructing a wave-plate that areexpected to withstand 105° C., such as evaporation of a film at anoblique angle on a substrate, but they are not currently available atlow enough cost to be used in a HUD.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention will be had uponreference to the following description in conjunction with theaccompanying drawings.

FIG. 1 is a schematic side view of one embodiment of an automotive headup display arrangement of the present invention.

FIG. 2 is a plan view of one embodiment of the polarizing device of theautomotive head up display arrangement of FIG. 1.

FIG. 3 is a flow chart of one embodiment of a head up display method ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates one embodiment of an automotive head up displayarrangement 10 of the present invention, including a HUD module 12 and awindshield 14. HUD module 12 includes a PGU in the form of LCD 16, apolarizing device 18, a first mirror 20, and a second mirror 22. LCD 16may transmit light 23 to polarizing device 18.

FIG. 2 illustrates one embodiment of polarizing device 18 including apolarization rotator having a stack of three linear polarizers or planepolarizers 24 a-c which may change the linear polarization direction oflight 23 after light 23 exits LCD 16. Each of polarizers 24 a-c may havea planar shape, and polarizers 24 a-c may be parallel to each other. Agap between the sequential layers provided by linear polarizers 24 a-bmay be filled with optically transparent material that matches the indexof refraction of the polarizer film. Also, a gap between the sequentiallayers provided by linear polarizers 24 b-c may be filled with opticallytransparent material that matches the index of refraction of thepolarizer film. Respective polarization directions of the threesuccessive polarizers 24 a-c may be oriented approximately at the sameincremental angle, as shown in FIG. 2.

An electric field direction of S-polarized light from LCD 16 isindicated by double arrow 26. An electric field direction after thelight passes through first polarizer 24 a is indicated by double arrow28. An electric field direction after the light passes through secondpolarizer 24 b is indicated by double arrow 30. An electric fielddirection after the light passes through third polarizer 24 c isindicated by double arrow 32.

The polarization rotator shown in FIG. 2 includes multiple layers ofplastic film linear polarizers. Each polarizer may be rotated byapproximately the same angle relative to the previous polarizer. Toavoid reflection loss at interfaces, the gap between sequential plasticfilm linear polarizers may be filled with an optically transparentmaterial that has an index of refraction that matches the index ofrefraction of the polarizer film. The shape of the layers is shown inFIG. 2 as rotated rectangles for purposes of illustration. In reality,once the layers are properly arranged, they may be fused together andcut to the desired shape. For a HUD, the layers may be cut to the shapeof the active area of the LCD.

The polarization rotator may maximize the fraction of incident lighttransmitted with the desired polarization direction. One source of lossis light reflected at interfaces. Typically, the total reflection lossfrom the front and back surfaces of individual plastic film polarizersis about 8%. For multiple polarizers stacked together, reflection lossbetween adjacent films can be eliminated by filling the gap between thelayers with a transparent material that has the same index ofrefraction.

During use, light 23 from LCD 16 may be polarized by polarizationrotator 18, and reflected by mirrors 20, 22 and windshield 14 toward auser 34. Light 23 may appear to user 34 as a virtual image 36 even whenuser 34 is wearing polarized sunglasses.

In another embodiment, two polarizers may be used. The first polarizermay be rotated 23 degrees, and the second polarizer may be rotated 46degrees. Thus, the polarization direction of the light after passingthrough both polarizers may be rotated 46 degrees. In a specificembodiment with an LCD display emitting white light, the illuminance maybe about 1037 cd/m². The color coordinates of the white light in CIE1931 color space may be about (x, y)=(0.304, 0.323). After passingthrough the two polarizers, the illuminance may be about 571 cd/m². Thecolor coordinates of the white light in CIE 1931 color space may beabout (x, y)=(0.318, 0.347). The transmittance of a single polarizer,oriented parallel to the direction of linear polarization, may be about89%. The loss due to reflection from the polarizer may be about 8%.

After passing light through a polarizer oriented to pass p-polarizedlight, the illuminance may be about 243 cd/m². After passing lightthrough a polarizer oriented to pass s-polarized light, the illuminancemay be about 224 cd/m². Thus, the ratio of p-polarized light tos-polarized light may be about 1.08.

In another specific embodiment, a 1.8 inch LCD is used in the HUD andhas an active area of 40.90×20.45 mm. The virtual image may provide abrightness of greater than 10,000 cd/m² for a white image vieweddirectly, and greater than 650 cd/m² for a white image viewed throughpolarized sunglasses. Thus, the fraction of light emitted as p-polarizedthat reaches the driver may be 6.5% or more. The angle of incidence ofthe light reaching the windshield may be about 64°. The index ofrefraction of the glass in the windshield may be about 1.52. At a 60degree angle of incidence, the windshield may reflect about 36.7% of thes-polarized light and about 0.305% of the p-polarized light. At a 65degree angle of incidence, the windshield may reflect 47.1% of thes-polarized light and 2.46% of the p-polarized light. Thus, afterreflection, the fraction of p-polarized light is reduced by a factor4.9% (2.46/(2.46+47.1)) relative to the s-polarized light. Consequently,the ratio of (p-polarized intensity)/(s-polarized intensity) exiting theLCD may be about 1.33 (6.5%/4.9%). Thus, the angle of the linearpolarization may be rotated by about 49°. This may be done with onesheet of half-wave retarder film rotated by 49°/2=24.5°. It may also bedone with three sheets of linear polarizer material, each rotated by16.3°. The half-wave retarder solution may transmit all of the incidentpower. The three-polarizer solution may transmit about 78% of theincident power.

FIG. 3 is a flow chart of one embodiment of a head up display method 300of the present invention. In a first step 302, a head up display moduleincluding a picture generation unit and a plurality of linear polarizersarranged in a stack is provided. For example, HUD module 12 includes aPGU and a polarizing device 18. Polarizing device 18 includes apolarization rotator having a stack of three linear polarizers or planepolarizers 24 a-c.

Next, in step 304, light is emitted by use of the picture generationunit. For example, PGU may be in the form of LCD 16 transmitting light23 to polarizing device 18.

In a next step 306, the light from the picture generation unit is passedsequentially through each of the linear polarizers to thereby change alinear polarization direction of the light. For example, two polarizersmay be used. The first polarizer may be rotated 23 degrees, and thesecond polarizer may be rotated 46 degrees. Thus, the polarizationdirection of the light after passing through both polarizers may berotated 46 degrees.

In a final step 308, the light that has passed through the linearpolarizers is reflected off of a windshield such that the reflectedlight is visible to a human driver of a motor vehicle as a virtualimage. For example, light 23 from LCD 16 may be polarized bypolarization rotator 18, and reflected by mirrors 20, 22 and windshield14 toward a user 34. Light 23 may appear to user 34 as a virtual image36 even when user 34 is wearing polarized sunglasses.

Different numbers of polarizers may be used within the scope of theinvention. Using fewer polarizer layers increases the loss in intensitycaused by light that is transmitted through one polarizer not beingtransmitted through the subsequent polarizer. Using many polarizersincreases the loss associated with absorption of light by thepolarizers.

The foregoing description may refer to “motor vehicle”, “automobile”,“automotive”, or similar expressions. It is to be understood that theseterms are not intended to limit the invention to any particular type oftransportation vehicle. Rather, the invention may be applied to any typeof transportation vehicle whether traveling by air, water, or ground,such as airplanes, boats, etc.

The foregoing detailed description is given primarily for clearness ofunderstanding and no unnecessary limitations are to be understoodtherefrom for modifications can be made by those skilled in the art uponreading this disclosure and may be made without departing from thespirit of the invention.

What is claimed is:
 1. A head up display module for a motor vehicle,comprising: a picture generation unit configured to emit a light field;and a plurality of linear polarizers arranged in a stack, a first of thelinear polarizers being configured to receive the light field from thepicture generation unit, a last of the linear polarizers beingconfigured to emit the light field, the linear polarizers beingconfigured to change a linear polarization direction of the light field,wherein each pair of adjacent said linear polarizers has a respectivegap between the pair of adjacent said linear polarizers, each said gapbeing filled with optically transparent material, and the opticallytransparent material matches an index of refraction of adjacent saidlinear polarizers.
 2. The head up display module of claim 1 whereinrespective polarization directions of the linear polarizers are orientedat approximately a same nonzero incremental angle.
 3. The head updisplay module of claim 1 wherein each of the linear polarizers has aplanar shape, and the linear polarizers are parallel to each other. 4.The head up display module of claim 1 wherein said linear polarizerscomprise plastic film.
 5. The head up display module of claim 1 whereinsaid picture generation unit comprises a liquid crystal display.
 6. Thehead up display module of claim 1 wherein the plurality of linearpolarizers comprise at least three linear polarizers.
 7. The head updisplay module of claim 1 wherein the last of the linear polarizers isconfigured to emit the light field such that the light field isreflected by a windshield of the motor vehicle and is visible to a humandriver of the motor vehicle as a virtual image.
 8. A head up displaymodule for a motor vehicle, comprising: a picture generation unitconfigured to emit light; and a plurality of linear polarizers havingoptically transparent material between adjacent ones of said linearpolarizers, wherein the optically transparent material matches an indexof refraction of adjacent said linear polarizers, the linear polarizersbeing configured to: receive the light from the picture generation unit;sequentially change a linear polarization direction of the receivedlight; and emit the light with a changed linear polarization direction.9. The head up display module of claim 8 wherein respective polarizationdirections of the linear polarizers are oriented at approximately a samenonzero incremental angle.
 10. The head up display module of claim 8wherein each of the linear polarizers has a planar shape, and the linearpolarizers are parallel to each other.
 11. The head up display module ofclaim 8 wherein said linear polarizers comprise plastic film.
 12. Thehead up display module of claim 8 wherein said picture generation unitcomprises a liquid crystal display.
 13. The head up display module ofclaim 8 wherein the plurality of linear polarizers comprise at leastthree linear polarizers.
 14. The head up display module of claim 8wherein the linear polarizers are configured to emit the light such thata windshield reflects the emitted light and such that the reflectedlight is visible to a human driver of the motor vehicle as a virtualimage.
 15. A head up display module for a motor vehicle, comprising: apicture generation unit configured to emit light; and a plurality oflinear polarizers having optically transparent material between adjacentones of said linear polarizers, wherein the optically transparentmaterial matches an index of refraction of adjacent said linearpolarizers, the linear polarizers being configured to: receive the lightfrom the picture generation unit; and emit the light with a changedlinear polarization direction.
 16. The head up display module of claim15 wherein respective polarization directions of the linear polarizersare oriented at approximately a same nonzero incremental angle.
 17. Thehead up display module of claim 15 wherein each of the linear polarizershas a planar shape, and the linear polarizers are parallel to eachother.
 18. The head up display module of claim 15 wherein said linearpolarizers comprise plastic film.
 19. The head up display module ofclaim 15 wherein the plurality of linear polarizers comprise at leastthree linear polarizers.
 20. The head up display module of claim 15wherein the linear polarizers are configured to emit the light such thata windshield reflects the emitted light and such that the reflectedlight is visible to a human driver of the motor vehicle as a virtualimage.